The present invention relates to transmission of electromagnetic radiation over optical fibers linking transmitter and receiver arrangements in the linking system and, more particularly, to the mating arrangements for components provided in such linking systems.
Fiber optic links are now widely implemented for communications and sensing systems. A block diagram representation of a typical fiber optic link system is shown in FIG. 1 having a driving circuit, 1, an optical emitter, 2, that are typically housed together on a support, 3, which are coupled to an optical fiber, 4. At the other end of optical fiber 4, that end is coupled to a photodetector, 5, and a receiving circuit, 6, that amplifies the signal received thereby from photodetector 5. Similarly, photodetector 5 and receiving circuit 6 are typically housed together on a second support, 7, using typical electronic device systems housing, or “packaging” techniques.
The most difficult requirements to be met in implementing the link system shown in FIG. 1 come from the need to couple light from optical emitter 2 into optical fiber 4, and to direct the light emerging from that fiber onto photodetector 5. The light carrying diameter of fiber 4 may range from approximately 8 μm for a glass fiber that supports propagation of a single electromagnetic radiation mode, to approximately 50 μm for a multimode glass fiber, to as much as 1 mm for a plastic fiber. The smaller diameter glass fiber, especially, requires proper optical lens design to efficiently couple light into the fiber, and precise alignment as well.
The block diagram of FIG. 1 illustrates a unidirectional link system, i.e. the information is transferred in one direction in being carried by the transmitted electromagnetic radiation. In communications applications, information transfer in the return direction is frequently addressed by a replication of this link system of FIG. 1, except that the information transfer is directed in the opposite direction.
However, there are situations in which there is a desire to be able to transfer information through a single optical fiber in both directions. For example, transceivers located on the edge of a printed circuit board based system are often limited by the amount of space available for connections thereto at the edge of the board. Bidirectional communication over a fiber would decrease the number of fibers required, and potentially double the number of bidirectional connections that fit in the limited space on the edge of that board. Alternatively, for longer distance connections, the cost of the optical fiber used becomes substantial. Reducing the number of fibers required by a factor of two creates a substantial cost reduction.
Further situations involving bidirectional radiations transmissions in a single optical fiber include optical sensing applications which often rely upon sending an electromagnetic radiation signal along an optical fiber that is subsequently modified by the interaction thereof with the material corresponding to the condition to be sensed, and then this modified radiation signal is reflected back along the same fiber for detection. Finally, monitoring of the physical conditions of the fiber optic link can be accomplished by means of optical time domain reflectometry (OTDR) methods.
In OTDR techniques, electromagnetic radiation transmitted along the optical fiber is backscattered or reflected at locations on the optical path where it encounters alterations in the index of refraction of this medium, and this radiation creates a return signal that is monitored over time. Such techniques provide quantitative information about the losses of radiation power occurring on the optical path in the fiber, and the losses thereof occurring at discrete locations along this optical path such as where the fiber is mated with a signal transfer connector.
FIG. 2 shows a graph having a plot demonstrating samples of the kinds of data that can be collected by these techniques. In this figure, the return signal radiation power versus time raw data has been translated into a plot of the return signal radiation power versus distance along the optical path based upon the speed of light in that medium. This return signal was acquired from a fiber optic link having two optical fibers of different lengths joined together with a signal transfer connector.
The plot shown of the resulting return signal has two line segment portions therein, 8 and 8′, each with a corresponding slope differing from that of the other, with a sharply changing plot portion, 9, (including an inflection point) between them involving air-fiber Fresnel reflection showing a discontinuity in the radiation backscattered that represents the result of the connector being used to join the fiber segments. Slope 8 of −9 db/km corresponds to a 100 m length step index fiber and slope 8′ of −63 db/km corresponds to a 200 meter length plastic clad silica fiber. The slopes of these two line segment plot portions allow estimating radiation power losses of the corresponding fibers. The magnitude of the plot portion change at the discontinuity allows estimating the radiation power loss occurring at the connection between the two fibers.
In long distance telecommunications systems based on fiber optic link systems, evaluating the performance and integrity of fiber optic links has usually been accomplished through use of optical time domain reflectometry. The long distances involved make very valuable the ability to identify remote disruption issues occurring along the optical path in the optical fiber or fibers used therein and the various junctions where joinings thereof occur and where also various signal taps provided. These issues can include fiber breaks, excessive fiber bends, and dirty or damaged fiber connectors. The ability to pinpoint the location of any such problems has significant benefits in the costs incurred in corresponding troubleshootings and repairs.
On the other hand, in shorter range data communications systems that are also based on fiber optic link systems where the links are typically on the order of 10 meters to 10 kilometers, the breakpoints in cost-benefit trade-offs have involved troubleshooting techniques having lower cost points. However, the “wiring closets”, or spaces where network equipment is located, are often cramped and stuffed to capacity with such equipment which could be alleviated to an extent by the use of bidirectional transmissions on individual optical fibers to thereby reduce the number of them used and the associated connectors and the like. The increasing density of components and their interconnections occurring in more recently provided ones of such wiring cabinets raises the costs of any troubleshooting undertaken with respect thereto.
Other fiber optic link systems uses have emerged, such as the implementation of fiber optical networks on military aircraft or vehicles, where the reliability of the network is essential. Military avionics environments typically pose more stringent requirements than commercial data transfer environments with regard to the integrity of the link and the ability to detect conditions that compromise that integrity. Gaining access to the various points of a link in a military aircraft, for instance, can be very difficult. Furthermore, there is a strong desire is to perform fairly thorough preventive maintenance procedures on such aircraft in between missions, and to perform them efficiently to permit the aircraft to be available for the next mission quickly. A high premium on personnel safety and mission success requires that the correct operation of a data communication link be of prime importance. Therefore, techniques that facilitate diagnosing problems in fiber optic communication links easily and quickly are much desired, and which can be significantly aided by the use of bidirectional transmissions on individual optical fibers to thereby reduce the number of them used and the associated supporting components.
Hence, there are numerous reasons for incorporating bidirectional signal transfer capability for individual optical fibers. This requires coupling electromagnetic radiation, or light, in and out of the individual optical fiber with an acceptably high coupling efficiency at an acceptable cost.
A significant factor in coupling such light in and out of individual optical fibers are the characteristics of the optoelectronic devices used in providing and detecting such light. The configurations of the optoelectronic device structures (structure top surface emitting or detecting arrangements, or structure side surface emitting or detecting arrangements), the symmetry and divergence of the light beams emitted from the light source, and the size of the detecting surface area of a photodetector are all necessary to be considered in selecting housing structures or packages to contain such optoelectronic devices and provide satisfactory optical coupling between them and the corresponding optical fiber.
FIG. 3 schematically illustrates some common configurations for semiconductor material chips capable of selectively providing electromagnetic radiation emissions as optical communication sources shown as projected above the semiconductor material wafers from the location therein at which they are fabricated using monolithic integrated circuit fabrication techniques. Almost all such optical communication sources used in long distance telecommunications fiber optics links are edge-emitting lasers (EELs). In these devices the light emissions come from the edge of the chip and tend to be asymmetric about a central beam direction axis with a large divergence (10° by 40°) from that axis in at least one plane therethrough. Light emitting diodes (LEDs) are surface emitters typically with a divergence equal to a full hemisphere and are used as optical communication sources only for relatively low data rate fiber optic links, and are typically being used only with plastic optical fibers in links provided in automobiles or industrial control networks.
Vertical cavity surface emitting lasers (VCSELs) are being used as optical communication sources for shorter distance data communication systems based on fiber optic link systems, that is, for link lengths shorter than 500 meters. They have the advantage of providing light emissions from a major surface of the chip such that these emissions form symmetric, low divergence beams. These lasers provide a combination of good performance at a reasonable cost for these shorter distance links, but typically do not provide sufficient performance for long distance telecommunication fiber optic links due to the mismatch between the laser emission wavelength and the optical fiber best transmission wavelengths (850 nm versus the desired 1310 nm or 1550 nm) and the wider laser emission spectral width as compared to the fiber transmission spectral widths at these wavelengths. However, longer wavelength VCSELs are currently under development.
The difficulties in coupling light from an optical fiber into photodetectors are typically less severe. Most photodetector semiconductor material chips detect light at a major surface therein, although a few special application detectors may require light to be coupled into the edge of the chip. Most significantly, the size of the active light detection region is usually related to the rapidity of the response of the device to impinging light, and so photodetectors for higher data rate links must correspondingly be smaller. This results in requiring more accurate alignment between fiber and that smaller photodetector to achieve a satisfactory optical coupling.
Furthermore, there is a desire to minimize the interaction, or “cross-talk”, between electromagnetic radiation transmitted in one direction through the link optical fiber and any aspects involving radiation propagation in the opposite direction in that fiber. For instance, reflected light from the fiber directed back into the aperture of the optical emitter can cause noise in that optical emitter thus degrading the information carrying capacity of the link.
Housings or packages are available for components used in coupling to optical fibers in bidirectional fiber optic links, including those that incorporate OTDR capability into a fiber optic transceiver, but they tend to be expensive, bulky and designed for use with EELs. For instance, planar lightwave circuits (PLCs) can be used to provide the input from the laser to an optical fiber and to also split off part of the return signal to a photodetector. PLCs based upon both silica material waveguides, and polymer material waveguides, have been applied in bidirectional fiber optic links for fiber-to-the-home (FTTH) signal distribution systems.
One arrangement for bidirectional coupling on fiber optic links using electromagnetic radiation transmissions of different wavelengths in each direction in an optical fiber is shown in FIG. 4. FIG. 4A is a schematic top view of the assembly for this arrangement. The electromagnetic radiation output from an edge-emitting laser diode, 2′, on a substrate, 3′,7′, is coupled into an optical waveguide, 4′, formed in or on that substrate. A spectral filter, 4″, reflects radiation at the wavelength of the laser and couples it into another waveguide segment, 4′″, in or on that substrate that is optically coupled to the transmission optical fiber 4. In the reverse direction along optical fiber 4, electromagnetic radiation transmitted from the opposite end of the optical fiber at a different wavelength is coupled from that fiber to that same optical waveguide 4′″. However, in this direction the wavelength of the radiation is such that it is transmitted by spectral filter 4″ into a photodetector, 5′, also provided on the substrate.
A side view in cross section of a portion of the assembly is shown in greater detail in FIG. 4B. Spectral filter 4″ is positioned in a 200 μm wide groove in substrate 3′,7′, typically of silicon. A 45° angle surface mirror across from this filter at a clad layer on the top of the substrate is used to reflect the incoming radiation upward to thereby couple it into the detecting area of a major separate parts and precise optical alignments between them, and the wavelengths of the radiations propagating in opposite directions in the fiber must be different.
A second arrangement for bidirectional coupling on fiber optic links is illustrated in FIG. 5 again using electromagnetic radiation transmissions of different wavelengths in each direction. In this arrangement, radiation at two wavelengths is propagated in one direction along the optical fiber, and radiation at a third wavelength is propagated in the opposite direction. There are two waveguide structure substrates, 3″, 7″, and 3′″, 7′″, one provided at each of the opposite ends of the link with link optical fiber 4 being coupled to each. Each of these waveguide structure substrates has a corresponding one of two fiber interface waveguide portions, 4″″ and 4v, on or in the substrate portion thereof coupled to a corresponding end of fiber 4, and extends in its waveguide structure substrate away from that fiber to a junction point at which it is split into three waveguide branches to direct radiation from a laser or lasers at one link end through the fiber to a photodetector or photodetectors at the opposite link end. Outgoing radiation in a waveguide branch from a laser, 2″, coupled thereto through a glass end plate in the corresponding waveguide structure substrate, is transmitted through the junction to the fiber interface waveguide portion that is coupled to fiber 4 with power losses being suffered at the junction. Incoming radiation is coupled from fiber 4 to the fiber interface waveguide portion and on to the junction where it is split into all three waveguide branches to be transmitted therethrough, and through the corresponding glass end plate in the waveguide structure substrate, to a photodetector 5″, or photodetectors but a spectral filter, 4vi, is again used to prevent such radiation from reaching any laser 2″. This arrangement can use either edge emitting or surface emitting devices, and either edge detecting or surface detecting devices, with each such optoelectronic device aligned to the appropriate waveguide branch. Once again, this arrangement requires many piece parts with precise alignment requirements, and radiation of different wavelengths for transmission in opposite directions in the fiber.
A third arrangement that does allow radiation of the same wavelength to be used in both directions in the link optical fiber involves using the same optoelectronic device as both an optical source and a photodetector. In particular, the use of a VCSEL both as a laser and as a photodetector has been demonstrated based on the active region of the VCSEL serving as a resonant cavity photodetector when reverse biased. Such devices can be coupled directly to the optical fiber in the link without requiring any additional waveguide interfaces. However, use of a VCSEL both as a laser and as a photodetector requires some (often unacceptable) compromises to be made in the device structure resulting in either an inferior laser, an inferior photodetector, or both. Thus, there is a desire for a bidirectional fiber optic coupler for coupling link fibers to corresponding transceivers.